Brain-wide changes in microglial morphology following peripheral nerve injury

Neuropathic pain involves disruptions in sensory, cognitive, and affective processing, with microglial reactivity playing a crucial role in its development. While spinal microglial changes post-injury are well-documented, the time-dependent patterns of microglial reactivity in the brain remain unclear. This study aimed to characterize microglial morphological changes over time following peripheral nerve injury. Adult male and female Sprague-Dawley rats underwent chronic constriction injury (CCI) or sham surgery to the sciatic nerve, and brains were collected at 7 or 28 days post-surgery. CD11b immunostaining was used to visualize microglia across 52 brain regions linked to sensory, affective, and cognitive pain modalities. Morphological measures-including reactivity score, area, and length-were quantified with HALO software. A repeated-measures linear mixed model revealed significant effects of injury and timepoint on all parameters. Post hoc analyses identified region-specific changes (FDR-adjusted) in areas such as the anterior cingulate cortex, central amygdala, dorsal raphe, and dorsomedial hypothalamus, highlighting spatial specificity of microglial responses. A functional circuit-wide correlation network analysis showed a dynamic reorganization of microglial morphology following injury. Initially, at day 7, the primary motor cortex emerged as a hub, reflecting acute sensorimotor changes. By day 28, network hubs had shifted to the dorsomedial hypothalamus and ventral tegmental area, suggesting engagement of homeostatic and reward circuits in chronic pain. Graph-theoretic metrics revealed a progressive decline in global network connectivity over time, supporting the view that chronic neuropathic pain alters central microglial signaling in a region- and circuit-specific manner, relevant to pain chronification and its comorbidities. PERSPECTIVE: This article presents a comprehensive mapping of microglial morphology in brain areas associated with pain processing. The findings will guide further investigations into the ways in which microglia contribute to neuropathic pain and its comorbidities.